Sending messages in an asynchronous manner avoids blocking the sending thread. This is great when your solution needs to scale in order to support a large number of clients, but there is a limit on how long can we wait for the asynchronous process to complete.

The Java WebSocket API gives you a few options in this regard.

Async Timeout Support

• First and foremost, there is a notion of a timeout. This can be configured using the setSendTimeout method in the RemoteEndpoint.Async interface.
• Secondly, the failure result manifests itself using the Future object or java.websocket.SendResult.

How Do Timeouts Manifest?

It depends on which strategy you’re using in order to send your messages:

• Callback-based.
• Future-based.

In case you are using the java.websocket.SendHandler, i.e. the callback handler route, the timeout exception details will be available via SendResult.getException(). 

....
public void broadcast(Session s, String msg){
  RemoteEndpoint asyncHandle = s.getRemoteAsync();
  asyncHandle.setSendTimeout(1000); //1 second
  asyncHandle.sendText(msg, 
    new SendHandler(){
      @Override
        public void onResult(SendResult result) {
        if(!result.isOK()){
        System.out.println("Async send failure: "+ result.getException());
        }
    }
  }); //will timeout after 2 seconds
  tracker.get(); //will throw java.util.ExecutionException if the process had timed out
}
....

If you chose to use the Future to track the completion, calling it get method will result in a java.util.concurrent.ExecutionException.

....
public void broadcast(Session s, String msg){
  RemoteEndpoint asyncHandle = s.getRemoteAsync();
  asyncHandle.setSendTimeout(2000); //2000 ms
  Future<Void> tracker = asyncHandle.sendText(msg); //will timeout after 2 seconds
  tracker.get(); //will throw java.util.ExecutionException if the process had timed out
}
....

Further Reading

• Java WebSocket API specification.
• Other WebSocket blogs.